U.S. patent number 7,204,970 [Application Number 10/730,630] was granted by the patent office on 2007-04-17 for single-wall carbon nanotubes from high pressure co.
This patent grant is currently assigned to William Marsh Rice University. Invention is credited to Robert K. Bradley, Michael J. Bronikowski, Daniel T. Colbert, Pavel Nikolaev, Frank Rohmund, Richard E. Smalley, Ken A. Smith.
United States Patent |
7,204,970 |
Smalley , et al. |
April 17, 2007 |
Single-wall carbon nanotubes from high pressure CO
Abstract
The present invention discloses the process of supplying high
pressure (e.g., 30 atmospheres) CO that has been preheated (e.g.,
to about 1000.degree. C.) and a catalyst precursor gas (e.g.,
Fe(CO).sub.5) in CO that is kept below the catalyst precursor
decomposition temperature to a mixing zone. In this mixing zone,
the catalyst precursor is rapidly heated to a temperature that
results in (1) precursor decomposition, (2) formation of active
catalyst metal atom clusters of the appropriate size, and (3)
favorable growth of SWNTs on the catalyst clusters. Preferably a
catalyst cluster nucleation agency is employed to enable rapid
reaction of the catalyst precursor gas to form many small, active
catalyst particles instead of a few large, inactive ones. Such
nucleation agencies can include auxiliary metal precursors that
cluster more rapidly than the primary catalyst, or through
provision of additional energy inputs (e.g., from a pulsed or CW
laser) directed precisely at the region where cluster formation is
desired. Under these conditions SWNTs nucleate and grow according
to the Boudouard reaction. The SWNTs thus formed may be recovered
directly or passed through a growth and annealing zone maintained
at an elevated temperature (e.g., 1000.degree. C.) in which tubes
may continue to grow and coalesce into ropes.
Inventors: |
Smalley; Richard E. (Houston,
TX), Smith; Ken A. (Katy, TX), Colbert; Daniel T.
(Houston, TX), Nikolaev; Pavel (Houston, TX),
Bronikowski; Michael J. (Pasadena, CA), Bradley; Robert
K. (Houston, TX), Rohmund; Frank (Huttlingen,
DE) |
Assignee: |
William Marsh Rice University
(Houston, TX)
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Family
ID: |
27493549 |
Appl.
No.: |
10/730,630 |
Filed: |
December 8, 2003 |
Prior Publication Data
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Document
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Publication Date |
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US 20040223901 A1 |
Nov 11, 2004 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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09830642 |
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6761870 |
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PCT/US99/25702 |
Nov 3, 1999 |
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60161728 |
Oct 27, 1999 |
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60117287 |
Jan 26, 1999 |
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60114588 |
Dec 31, 1998 |
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60106917 |
Nov 3, 1998 |
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Current U.S.
Class: |
423/447.2;
977/750; 977/751; 423/447.3 |
Current CPC
Class: |
B82Y
30/00 (20130101); B01J 3/04 (20130101); B01J
4/002 (20130101); B82Y 40/00 (20130101); D01F
9/1278 (20130101); B01J 19/121 (20130101); C01B
32/162 (20170801); B01J 3/042 (20130101); B01J
19/26 (20130101); Y10S 977/951 (20130101); Y10S
977/75 (20130101); C01B 2202/36 (20130101); B01J
2219/0875 (20130101); C01B 2202/02 (20130101); Y10S
977/843 (20130101); Y10S 977/845 (20130101); Y10T
428/30 (20150115); Y10S 977/751 (20130101) |
Current International
Class: |
C01B
11/02 (20060101) |
Field of
Search: |
;977/750,751
;423/447.2,447.3 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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2 248 230 |
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Apr 1992 |
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GB |
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06322615 |
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Nov 1994 |
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JP |
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09188509 |
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Jul 1997 |
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JP |
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WO 89/07163 |
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Aug 1989 |
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WO |
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WO 97/09272 |
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Mar 1997 |
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WO |
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WO 98/05920 |
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Feb 1998 |
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WO |
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WO 99/06618 |
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Feb 1999 |
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WO |
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WO 00/17102 |
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Mar 2000 |
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WO |
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WO 00/73205 |
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Jul 2000 |
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WO |
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|
Primary Examiner: Hendrickson; Stuart L.
Assistant Examiner: Fiorito; James
Attorney, Agent or Firm: Fish & Richardson P.C. Garsson;
Ross Spencer
Parent Case Text
RELATED APPLICATIONS
This application is a division of co-pending prior division
application Ser. No. 09/830,642, filed Jul. 1, 2002, which is the
35 U.S.C. .sctn. 371 national application of International
Application Number PCT/US99/25702 filed on Nov. 3, 1999, which
designated the United States, claiming priority to provisional U.S.
patent application Ser. Nos. 60/106,917, filed on Nov. 3, 1998;
60/114,588, filed Dec. 31, 1998; 60/117,287, filed Jan. 26, 1999;
and 60/161,728, filed Oct. 27, 1999.
Claims
What is claimed is:
1. A composition of matter comprising single wall carbon nanotubes
wherein at least 95% of said single wall carbon nanotubes have a
diameter in the range of 0.6 nm to 0.8 nm.
2. The composition of matter of claim 1, wherein said single wall
carbon nanotubes are aggregated as ropes.
3. The composition of matter of claim 1, wherein said single wail
carbon nanotubes comprise (5,5) single wall carbon nanotubes.
4. A product made by a process comprising: (a) providing a gas
stream comprising CO at superatmospheric pressure; (b) providing a
gaseous catalyst precursor stream comprising a gaseous catalyst
precursor comprising atoms of a transition metal selected from the
group consisting of Group VI metals, Group VIII metals and mixtures
thereof, said gaseous catalyst precursor stream being provided at a
temperature below the decomposition temperature of said catalyst
precursor; (c) heating said gas stream comprising CO to a
temperature that is at least above the decomposition temperature of
said catalyst precursor and is sufficient to form single wall
carbon nanotubes; (d) mixing said heated gas stream comprising CO
with said gaseous catalyst precursor stream into a reaction mixture
in a mixing zone to rapidly heat said catalyst precursor to a
temperature that is (i) above the decomposition temperature of said
catalyst precursor, (ii) sufficient to promote the formation of
catalyst metal atom clusters and (iii) sufficient to promote the
initiation and growth of single wall carbon nanotubes; and (e)
forming solid products comprising the single wall carbon nanotubes
that are in a resulting gaseous stream, wherein at least 99% of
atoms of the solid products are atoms of the single wall carbon
nanotubes.
5. A product made by a process comprising: (a) providing a gas
stream comprising CO at superatmospheric pressure; (b) providing a
gaseous catalyst precursor stream comprising a gaseous catalyst
precursor comprising atoms of a transition metal selected from the
group consisting of Group VI metals, Group VIII metals and mixtures
thereof, said gaseous catalyst precursor stream being provided at a
temperature below the decomposition temperature of said catalyst
precursor; (c) heating said gas stream comprising CO to a
temperature that is at least above the decomposition temperature of
said catalyst precursor and is sufficient to form single wall
carbon nanotubes; (d) mixing said heated gas stream comprising CO
with said gaseous catalyst precursor stream into a reaction mixture
in a mixing zone to rapidly heat said catalyst precursor to a
temperature that is (i) above the decomposition temperature of said
catalyst precursor; (ii) sufficient to promote the formation of
catalyst metal atom clusters and (iii) sufficient to promote the
initiation and growth of single wall carbon nanotubes; and (e)
forming solid products comprising the single wall carbon nanotubes
that are in a resulting gaseous stream, wherein at least 95% of
said single wall carbon nanotubes have a diameter in the range of
0.6 nm to 0.8 nm.
6. A product made by a process comprising: (a) providing a CO gas
stream comprising CO, wherein said CO gas stream is at a
superatmospheric pressure; (b) providing a gaseous catalyst
precursor stream comprising a catalyst precursor; (c) mixing the CO
gas stream and the gaseous catalyst precursor stream to form a
reaction mixture, wherein said mixing step occurs under reaction
conditions to form single wall carbon nanotubes; and (d) reacting
said reaction mixture to form carbon products in tubular form,
wherein at least 99% atom % of the carbon products in tubular form
are single wall carbon nanotubes.
7. The product of claim 6, wherein at least 75% of said single wall
carbon nanotubes have a diameter in the range of 0.6 nm to 0.8
nm.
8. The product of claim 6, wherein at least 95% of said single wall
carbon nanotubes have a diameter in the range of 0.6 nm to 0.8 nm.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to the production of single-wall
nanotubes; in particular, to gas-phase nucleation and growth of
single-wall carbon nanotubes from high pressure CO.
2. Description of Related Art
Fullerenes are closed-cage molecules composed entirely of
sp.sup.2-hybridized carbons, arranged in hexagons and pentagons.
Fullerenes (e.g., C.sub.60) were first identified as closed
spheroidal cages produced by condensation from vaporized
carbon.
Fullerene tubes are produced in carbon deposits on the cathode in
carbon arc methods of producing spheroidal fullerenes from
vaporized carbon. Ebbesen et al. (Ebbesen I), "Large-Scale
Synthesis Nanotubes," Nature, Vol. 358, p. 220 (Jul. 16, 1992) and
Ebbesen et al., (Ebbesen II), "Carbon Nanotubes," Annual Review of
Materials Science, Vol. 24, p. 235 (1994). Such tubes are referred
to herein as carbon nanotubes. Many of the carbon nanotubes made by
these processes were multi-wall nanotubes, i.e., the carbon
nanotubes resembled concentric cylinders. Carbon nanotubes having
up to seven walls have been described in the prior art. Ebbesen II;
Iijima et al., "Helical Microtubules Of Graphitic Carbon," Nature,
Vol. 354, p. 56 (Nov. 7, 1991).
Single-wall carbon nanotubes have been made in a DC arc discharge
apparatus of the type used in fullerene production by
simultaneously evaporating carbon and a small percentage of Group
VIII transition metal from the anode of the arc discharge
apparatus. See Iijima et al., "Single-Shell Carbon Nanotubes of 1
nm Diameter," Nature, Vol. 363, p. 603 (1993); Bethune et al.,
"Cobalt Catalyzed Growth of Carbon Nanotubes with Single Atomic
Layer Walls," Nature, Vol. 363, p. 605 (1993); Ajayan et al.,
"Growth Morphologies During Cobalt Catalyzed Single-Shell Carbon
Nanotube Synthesis," Chem. Phys. Lett., Vol. 215, p. 509 (1993);
Zhou et al., "Single-Walled Carbon Nanotubes Growing Radially From
YC.sup.2 Particles," Appl. Phys. Lett., Vol. 65, p. 1593 (1994);
Seraphin et al., "Single-Walled Tubes and Encapsulation of
Nanocrystals Into Carbon Clusters," Electrochem. Soc., Vol. 142, p.
290 (1995); Saito et al., "Carbon Nanocapsules Encaging Metals and
Carbides," J. Phys. Chem. Solids, Vol. 54, p. 1849 (1993); Saito et
al., "Extrusion of Single-Wall Carbon Nanotubes Via Formation of
Small Particles Condensed Near an Evaporation Source," Chem. Phys.
Lett., Vol. 236, p. 419 (1995). It is also known that the use of
mixtures of such transition metals can significantly enhance the
yield of single-wall carbon nanotubes in the arc discharge
apparatus. See Lambert et al., "Improving Conditions Toward
Isolating Single-Shell Carbon Nanotubes," Chem. Phys. Lett., Vol,
226, p. 364 (1994). High quality single-wall carbon nanotubes have
also been generated by arc evaporation of a graphite rod doped with
Y and Ni. See C. Journet et al., Nature 388 (1997) 756, hereby
incorporated by reference in its entirety. These techniques allow
production of only gram quantities of single-wall carbon nanotubes
at low yield of nanotubes and the tubes exhibit significant
variations in structure and size between individual tubes in the
mixture.
An improved method of producing single-wall nanotubes is described
in U.S. patent application Ser. No. 08/687,665, entitled "Ropes of
Single-Walled Carbon Nanotubes" incorporated herein by reference in
its entirety. This method uses, inter alia, laser vaporization of a
graphite substrate doped with transition metal atoms, preferably
nickel, cobalt, or a mixture thereof, to produce single-wall carbon
nanotubes in yields of at least 50% of the condensed carbon. See A.
Thess et al., Science 273 (1996) 483; T. Guo., P. Nikolaev, A.
Thess, D. T. Colbert, R. E. Smalley, Chem. Phys. Lett., 243, 49 54
(1995), both incorporated herein by reference. The single-wall
nanotubes produced by this method tend to be formed in clusters,
termed "ropes," of 10 to 1000 single-wall carbon nanotubes in
parallel alignment, held together by van der Waals forces in a
closely packed triangular lattice. Nanotubes produced by this
method vary in structure, although one structure tends to
predominate. These high quality samples have for the first time
enabled experimental confirmation of the structurally dependent
properties predicted for carbon nanotubes. See J. W. G. Wildoer, L.
C. Venema, A. G. Rinzler, R. E. Smalley, C. Dekker, Nature, 391
(1998) 59; T. W. Odom, J. L. Huang, P. Kim, C. M. Lieber, Nature,
391 (1998) 62. Although the laser vaporization process produces
improved single-wall nanotube preparations, the product is still
heterogeneous, and the nanotubes are too tangled for many potential
uses of these materials. In addition, the vaporization of carbon is
a high energy process and is inherently costly.
Another known way to synthesize nanotubes is by catalytic
decomposition of a carbon-containing gas by nanometer-scale metal
particles supported on a substrate. The carbon feedstock molecules
decompose on the particle surface, and the resulting carbon atoms
then diffuse through the particle and precipitate as a part of
nanotube from one side of the particle. This procedure typically
produces imperfect multiwalled nanotubes in high yield. See C. E.
Snyder et al., Int. Pat. WO 9/07163 (1989), hereby incorporated by
reference in its entirety.
Yet another method for production of single-wall carbon nanotubes
involves the disproportionation of CO to form single-wall carbon
nanotubes+CO.sub.2 on alumina-supported transition metal particles
such as Mo, Mo/Fe, and Ni/Co. See Dai, H. J. et al., "Single-Wall
Nanotubes Produced by Metal-Catalyzed Disproportionation of Carbon
Monoxide," Chem. Phys. Lett., 1996. 260 (3 4): p. 471 475. In this
process the transition metal particles on the alumina support that
were large enough to produce multi-walled nanotubes were
preferentially deactivated by formation of a graphitic overcoating,
leaving the smaller metal particles to catalyze the growth of
single-wall carbon nanotubes. Good quality single-wall carbon
nanotubes can be grown from alumina-supported catalysts even with
hydrocarbon feed stocks such as ethylene, provided the multi-walled
nanotube production is suppressed by a pretreatment process. See
Hafner, H. F. et al., "Catalytic Growth of Single-Wall Carbon
Nanotubes From Metal Particles," Chem. Phys. Lett., 1998. 296 (1
2): p. 195 202; and U.S. Provisional Patent Application No.
60/101,093, entitled "Catalytic Growth of Single Wall Carbon
Nanotubes from Metal Particles," and International Application No.
PCT/US99/21367, hereby incorporated by reference in their entirety.
These methods use cheap feed stocks in a moderate temperature
process. Their yield is intrinsically limited due to rapid
surrounding of the catalyst particles and the alumina particle that
supports them by a dense tangle of single-wall carbon nanotubes.
This tangle acts as a barrier to diffusion of the feedstock gas to
the catalyst surface, inhibiting further nanotube growth. Removal
of the underlying alumina support from the nanotubes that form
around it will be an expensive process step.
Hollow carbon fibers that resemble multi-walled carbon nanotubes
have been produced from entirely gas phase precursors for several
decades. See Dresselhaus, M. S., G. Dresselhaus, and P. C. Ecklund,
Science of Fullerenes and Carbon Nanotubes, 1996, San Diego:
Academic Press, 985. Endo first used ferrocene and benzene vapors
traveling through a quartz tube in an Ar+H.sub.2 carrier gas at
about 1000.degree. C. to make carbon nanotubes (imperfect
multi-walled carbon nanotubes) overcoated in a largely amorphous
carbon. See Endo, M., "Grow carbon fibers in the vapor phase,"
Chemtech, 1988: p. 568 576. Tibbetts has used ferrocene and iron
pentacarbonyl to produce similar hollow carbon fibers from
methane/hydrogen mixtures at 1000.degree. C., a process that he
finds is benefited by the addition of sulfur in the form of
H.sub.2S. See Tibbetts, G. G., "Vapor-Grown Carbon Fibers: Status
and Prospects. Carbon," 1989. 27(5): p. 745 747. In some of Endo's
early experiments it is clear that small amounts of single-wall
carbon nanotubes were produced as well. But until recently no means
has been found to adapt these gas phase methods to produce
primarily single-wall carbon nanotubes.
Very recently it has been found that control of the
ferrocene/benzene partial pressures and addition of thiophene as a
catalyst promoter in the all gas-phase process can produce
single-wall carbon nanotubes. See Sen, R. et al., "Carbon Nanotubes
By the Metallocene Route," Chem. Phys. Lett., 1997 267(3 4): p. 276
280; Cheng, H. M. et al., "Large-Scale and Low-Cost Syntheses of
Single-Wall Carbon Nanotubes By the Catalytic Pyrolysis of
Hydrocarbons," Appl. Phys. Lett., 1998. 72(25): p. 3282 3284;
Dresselhaus, M. S., "Carbon Nanotubes--Introduction," Journal of
Materials Research, 1998. 13(9): p. 2355 2356. However, all these
methods involving hydrocarbon feed stocks suffer unavoidably from
the simultaneous production of multi-walled carbon nanotubes,
amorphous carbon, and other products of hydrocarbon pyrolysis under
the high temperature growth conditions necessary to produce high
quality single-wall carbon nanotubes.
Therefore, there remains a need for improved methods of producing
singlewall nanotubes of greater purity and homogeneity.
SUMMARY OF THE INVENTION
The present invention provides a method and apparatus for the
efficient, industrial scale production of single-wall carbon
nanotubes (SWNTs) from all gaseous reactants and which product is
substantially free of solid contaminants or by-products (e.g.1,
amorphous carbon deposits). This process is based on the use of
high pressure CO as the carbon source and an appropriate gaseous
transition metal catalyst precursor.
The present invention provides a method for producing single wall
carbon nanotube products comprising the steps of: (a) providing a
high pressure CO gas stream; (b) providing a gaseous catalyst
precursor stream comprising a gaseous catalyst precursor that is
capable of supplying atoms of a transition metal selected from
Group VI, Group VIII or mixture thereof, said gaseous catalyst
precursor stream being provided at a temperature below the
decomposition temperature of said catalyst precursor; (c) heating
said high pressure CO gas stream to a temperature that is (i) above
the decomposition temperature of said catalyst precursor and (ii)
above the minimum Boudouard reaction initiation temperature, to
form a heated CO gas stream; and (d) mixing said heated CO gas
stream with said gaseous catalyst precursor stream in a mixing zone
to rapidly heat said catalyst precursor to a temperature that is
(i) above the decomposition temperature of said catalyst precursor,
(ii) suffcient to promote the rapid formation of catalyst metal
atom clusters and (iii) sufficient to promote the initiation and
growth of single-wall nanotubes by the Boudouard reaction, to form
a suspension of single wall carbon nanotube products in the
resulting gaseous stream.
The present invention also provides an apparatus for producing
single wall carbon nanotube products comprising: (a) a high
pressure reaction vessel comprising in serial communication a
reactant introduction in zone, a reactant mixing zone, a growth and
annealing zone and a product recovery zone; (b) a first reactant
supply conduit for supplying a heated high pressure CO gas to said
introduction zone; (c) a second reactant supply conduit for
supplying a gaseous catalyst precursor to said information zone;
(d) mixing means for rapidly and intimately mixing the gas flows
from said first and second reactant supply conduits as said flows
enter said mixing zone; (e) heating means for maintaining said
growth and annealing zone at an elevated temperature; and (f)
gas/solids separation means positioned in said product recovery
zone to remove solid single wall carbon nanotube products from the
gas flows exiting said growth and annealing zone.
The present invention further provides a composition of matter
comprising single-wall carbon nanotubes having a tube diameter in
the range of 0.6 nm to 0.8 nm.
The present invention further provides a composition of matter
comprising (5,5) single-wall carbon nanotubes.
The process involves supplying high pressure (e.g., 30 atmospheres)
CO that has been preheated (e.g., to about 1000.degree. C.) and a
catalyst precursor gas (e.g., Fe(CO).sub.5) in CO that is kept
below the catalyst precursor decomposition temperature to a mixing
zone. In this mixing zone, the catalyst precursor is rapidly heated
to a temperature that results in (1) precursor decomposition, (2)
formation of active catalyst metal atom clusters of the appropriate
size, and (3) favorable growth of SWNTs on the catalyst clusters.
Preferably a catalyst cluster nucleation agency is employed to
enable rapid reaction of the catalyst precursor gas to form many
small, active catalyst particles instead of a few large, inactive
ones. Such nucleation agencies can include auxiliary metal
precursors that cluster more rapidly than the primary catalyst, or
through provision of additional energy inputs (e.g., from a pulsed
or CW laser) directed precisely at the region where cluster
formation is desired. Under these conditions SWNTs nucleate and
grow according to the Boudouard reaction. The SWNTs thus formed may
be recovered directly or passed through a growth and annealing zone
maintained at an elevated temperature (e.g., 1000.degree. C.) in
which tubes may continue to grow and coalesce into ropes.
The SWNT products can be separated from the gaseous stream and
recovered. The gaseous stream, which is primarily unreacted CO can
be recovered and recycled. The resulting SWNT products are
substantially pure (99%) and can be used without complicated
separation and purification steps. The process of this invention
also provides the ability to reproducibly control the diameter of
SWNT products produced. This process also provides the first SWNT
process that can produce a product that is substantially made up of
small diameter nanotubes (e.g., (5,5) tubes).
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic representation of one form of the process of
the present invention.
FIG. 2 shows the pressure vessel and oven within the apparatus
useful to perform the process of the present invention.
FIG. 3 shows another arrangement of the apparatus useful to perform
the process of the present invention.
FIG. 4 shows introduction of a laser beam in the reagent mixing
section of the apparatus useful to perform the process of the
present invention.
FIG. 5 is a schematic representation of an alternative process for
gas-phase nucleation and growth of single-wall carbon nanotubes
from high pressure CO according to another embodiment of the
present invention.
FIG. 6 is a schematic representation of an alternative process for
gas-phase nucleation and growth of single-wall carbon nanotubes
from high pressure CO according to another embodiment of the
present invention.
FIG. 7 is a schematic representation of an alternative process for
gas-phase nucleation and growth of single-wall carbon nanotubes
from high pressure CO according to another embodiment of the
present invention.
FIG. 8 is a series of photomicrographs showing the SWNT product
produced according to the process of the present invention in which
FIG. 8(a) is a TEM and FIG. 8(b) is a SEM.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
Carbon has, from its very essence, not only the propensity to
self-assemble from a high temperature vapor to form perfect
spheroidal closed cages (of which C.sub.60 is prototypical), but
also (with the aid of a transition metal catalyst) to assemble into
perfect single-wall cylindrical tubes which may be sealed perfectly
at one or both ends with a semifullerene dome. These tubes, which
may be thought of as one-dimensional single crystals of carbon, are
true fullerene molecules.
Single-wall nanotubes are much more likely to be free of defects
than multi-wall carbon nanotubes. Defects in single-wall carbon
nanotubes are less likely than defects in multi-walled carbon
nanotubes because the latter can survive occasional defects, while
the former have no neighboring walls to compensate for defects by
forming bridges between unsaturated carbon valances. Since
single-wall carbon nanotubes will have fewer defects, they are
stronger, more conductive, and therefore more useful than
multi-wall carbon nanotubes of similar diameter.
Raw Materials
1. Carbon Source
The primary carbon source employed in the process of the present
invention is carbon monoxide. CO is a readily available industrial
gas that can be used with minimal pretreatment in the process of
the present invention. Typically, filtration to remove unwanted
particulate contaminants is all that is required. Alternatively, if
desired, other purification processes such as sorption can be
employed to remove unwanted gaseous contaminants in the CO
feedstock. As described in more detail below, a major portion of
the CO feed gas stream may result from recycling the gaseous
effluent from the process.
3. Catalyst Precursor
Single-wall nanotube formation is known to be catalyzed by small
metal clusters that reside at the "growing" end of the tube, and
act to promote reactions in which a carbon-bearing feedstock is
converted to carbon in the form of a single-wall nanotube.
According to the present invention, a gaseous catalyst precursor
from which the catalyst cluster forms may be a metal-containing
compound that is in the gaseous state under the reaction
conditions.
As described below the size of this catalyst metal atom cluster has
an important influence on the nature of the product produced and in
the selectivity of the process to produce SWNTs. Useful metals
include the Group VI and/or Group VIII transition metals and
combinations thereof Suitable metals include tungsten, molybdenum,
chromium, iron, nickel, cobalt, rhodium, ruthenium, palladium,
osmium, iridium, platinum, and mixtures thereof Generally preferred
are catalyst systems based on Fe, or Co. The preferred catalyst
precursor compounds are metal carbonyls (e.g., Fe(CO).sub.5,
Co(CO).sub.6). Metallocene precursors such as FeCp.sub.2,
COCP.sub.x can also be used.
4. Nucleating Agents
As described in greater detail below, the process of the present
invention is based in part on the provision of rapid (near
simultaneous) (1) formation of the active catalyst metal atom
cluster of the appropriate size and (2) initiation of SWNT growth.
In order to form clusters of Fe atoms from dissociated precursor
molecules (e.g., Fe(CO).sub.5), the cluster must grow to the
minimum nucleation size, typically 4 5 atoms. Aggregation is
effected at this early stage by how tightly the COs are bound to
the Fe atom in the precursor and how tightly the initially formed
Fe dimer is bound. The Fe dimer binding energy is relatively low
(on the order of 1 eV). Accordingly the formation of Fe atom
aggregates of 4 5 atoms is a bit sluggish at the reaction
temperatures (800 1000.degree. C.). More rapid nucleation can be
effected by including a nucleating agent in the gas feed stream.
Such a nucleating agent can be a precursor moiety that under the
reaction conditions stimulates clustering by decomposing more
rapidly or binding to itself more tightly after dissociation. One
such nucleating agent that has been shown to substantially improve
the performance of Fe catalysts is Ni(CO).sub.4. The binding energy
of the Ni dimer is on the order of 2 eV and thus Ni dimers are more
likely than Fe dimers to facilitate rapid aggregation to the
critical 4 or 5 atom cluster level. Fe atom clusters thus may be
formed homogeneously or on seed clusters of Ni atoms. Any
metal-containing precursor that facilitates this rapid nucleation
can be employed. Other suitable examples include Mo(CO).sub.6 and
W(CO).sub.6. In the case of the Fe/Ni system, ratios of
Fe(CO).sub.5 to Ni(CO).sub.4 can range from about 10:1 to about 1:2
on an atom basis. Preferred are ratios in the range of about 3:1 to
1:1 with the most preferred ratio being about 1:1.
The use of nucleating agents can increase the productivity of the
process significantly (e.g., 2 4 or more times). This increase is
especially unexpected since Ni(CO).sub.4 alone has no appreciable
catalytic effect in the high pressure CO process of the present
invention under conditions typically employed.
Process Description
As shown in FIG. 1, one embodiment of the overall process of the
present invention involves the supply of high pressure CO from a
suitable source shown here as CO supply vessel 1. After optional
cleanup in filtration unit 2, the high pressure CO is divided into
undiluted stream 3 and catalyst carrier stream 4. An additional
stream 3' may also be provided. Catalyst precursor is supplied via
stream 5 from a suitable source, shown here as catalyst supply
vessel 6. A catalyst-containing CO stream 7 is then formed by
combining streams 4 and 5.
The gas phase process of the present invention operates at high
(i.e., superatmospheric) pressure. Since the gaseous reactants are
predominantly CO, the reaction pressure parameters can be best
discussed in terms of the partial pressure of CO, i.e., P.sub.CO.
In general, it is preferred to employ P.sub.CO in the range of from
about 3 to about 1000 atm. More preferred are P.sub.CO values in
the range of about 5 to 500 atm, with most preferred values being
in the range of 10 to 100 atm. In general, higher P.sub.CO values
in these ranges are preferred. As the P.sub.CO of the reaction is
increased, at least three benefits are achieved. Firstly, the
partial pressure of catalyst precursor P.sub.CAT can be increased
as the P.sub.CO is increased resulting in more catalyst clusters
and better productivity. Secondly, the Boudouard reaction is faster
at higher pressures and this facilitates the rapid growth of SWNTs.
Finally, at higher P.sub.CO values the catalyst precursor (e.g.,
Fe(CO).sub.5) decomposition temperature becomes closer to the
optimum nanotube growth temperatures, thus facilitating faster
cluster growth and the desired relatively simultaneous cluster
formation and growth reactions.
The concentration of catalyst precursor in the total CO gas feed
should be in the range of from about 1 to 100 ppm, and preferably
about 5 to 50 ppm. Typical concentrations in the range of 10 30 ppm
may be employed in a most preferred embodiment of the process. It
is convenient to refer to the catalyst precursor feed concentration
in terms of its partial pressure, P.sub.CAT. This value can in
general range from about 250 mTorr up to 100 Torr. As described
above, higher P.sub.CAT values can advantageously be employed as
P.sub.CO is increased. Preferred P.sub.CAT ranges are from 0.5 Torr
to 50 Torr, with more preferred values ranging from about 1 to 10
Torr.
While flow rate necessary to achieve the partial pressures
described above will vary with the particular design and scale of
the apparatus employed, typical flow rates for the apparatus
schematically shown in FIG. 1 are on the order of 1 slm of catalyst
precursor stream 5, 0 20 slm for CO dilutions stream 4 and 0 150
slm for undiluted CO stream 3.
Catalyst-containing stream 7 and undiluted CO stream 3 are
forwarded to a mixing zone 8. Although not shown in this figure and
as described in more detail below, stream 3 should be preheated
prior to or in combination with its introduction into the mixing
zone. Any suitable means normally employed to preheat gas streams
may be employed.
The preheating of undiluted CO stream 3 generally should be
sufficient to result in a reaction mixture, after combining with
catalyst precursor/CO stream 7, that is rapidly and uniformly
heated to a temperature that favors near simultaneous catalyst
cluster formation and SWNT growth via the Boudouard reaction. This
reaction temperature should be in the range of about 850.degree. C.
to 1250.degree. C. Accordingly, CO stream 3 generally is heated to
the range of from about 850.degree. C. to 1500.degree. C.
Preferably this preheating step results in a CO stream 3
temperature in the range of about 900 1100.degree. C. with the most
preferred temperature being about 1000 C. Stream 7 should be kept
below the decomposition temperature of the catalyst precursor. This
can be accomplished, if necessary, by using known cooling methods
such as air or water cooling. Preferably the catalyst/CO stream 7
is kept at a temperature below 200.degree. C., and preferably is
maintained at a temperature in the range of from about 70.degree.
C. to 200.degree. C. If the temperature exceeds the catalyst
decomposition temperature, clusters may form too early in the
process and become inactivated before they can participate in the
SWNT growth process. Of course, the temperature range may vary
depending on the precise catalyst or catalyst mixture employed.
Streams 3 and 7 are then combined in mixing zone 8 where nucleation
and growth of SWNTs take place. The mixing zone 8 should be
configured to provide rapid mixing of preheated CO stream with
catalyst precursor containing stream 7. As this mixing takes place,
the catalyst precursor stream is rapidly heated to a temperature in
the range of from about 900 1000.degree. C. in one preferred
embodiment. Extremely short mixing times are desired and can be
referred to as nearly simultaneous. These mixing times should
preferably be below about 1 msec and preferably on the order of 1
to 100 usec. The object of this fast mixing is the fast and uniform
heating of the catalyst precursor. Accordingly, turbulent mixing
conditions are preferred since heat transfer is promoted thereby.
As a result of these rapid mixing conditions, the volume of the
mixing zone will not be large. Typically, complete mixing/heating
is accomplished in a volume on the order of 1 cm or less. Flow
rates to the mixing zone can be controlled for a given mixing zone
configuration to provide the requisite turbulence and are typically
subsonic although supersonic mixing may be employed.
The mixture of SWNTs freely suspended in gas leaving the mixing
zone enters growth and annealing zone 9. This zone is preferably
kept at an elevated temperature by enclosing it in an oven 10,
containing heating elements 11 of any suitable kind. The oven 10 is
preferably maintained at a temperature of from about 850.degree. C.
to 1250.degree. C. and more preferably is maintained at a
temperature of about 1000.degree. C. The oven 10 is preferably
supplied with a pressure equalizing gas, e.g., N.sub.2 from supply
vessel 12. This gas should equal or slightly exceed the operating
pressure in the system. In the growth and annealing zone,
additional growth of previously formed SWNTs may take place, as may
the formation of new tubes. In this zone, the formed tubes may also
aggregate and remain bound to one another by van der Waals forces
to form ropes (i.e., up to about 10.sup.3 or more tubes in
generally parallel alignment).
After leaving growth and annealing zones, the mixture of gas
(primarily unreacted CO and CO.sub.2) containing suspended SWNT
products (mostly ropes) is forwarded to a product recovery zone 12.
In the product recovery zone, the solid product 13 is removed from
the gas stream by any suitable means and the separated gas stream
14 can be recycled. Product separation can be accomplished by any
known gas/solids separation means including filtration or the like.
To facilitate continuous operation, an endless belt or drum-type
filter carrier can be employed in a known manner.
Recycle gas stream 14 can be forwarded to supply vessel 1.
Preferred intermediate steps can include CO.sup.2 removal at 15 and
storage in low-pressure supply vessel 16. The low pressure CO can
be recompressed with any suitable means shown at 17 and then
forwarded to high-pressure storage vessel 1.
SWNT Diameter Control
One important aspect of the process of the present invention is the
ability to control the tube diameter of the SWNTs produced.
Generally, the diameter of the growing nanotube is proportional to
the size of its active catalyst cluster at the time the tube starts
to grow. The factors that control tube diameter include the rate of
aggregation of metal particles to form catalyst clusters and the
rate at which nanotube growth begins upon a cluster of given size.
The relationship of these two rates can be controlled in three ways
that can be used separately or together as desired. The first
control mechanism involves the ratio of P.sub.CO to P.sub.CAT.
Larger P.sub.CO/P.sub.CAT ratios result in smaller catalyst metal
atom clusters which provide smaller diameter tubes. Conversely
lower P.sub.CO/P.sub.CAT ratios result in rapid formation of larger
metal clusters which produce larger diameter tubes. Even for a
constant value of P.sub.CO/P.sub.CAT, higher absolute values of
P.sub.CO result in formation of smaller tubes, because the
initiation of tube growth takes place more effectively at higher
pressures of CO. Stated another way, at a fixed temperature and
metal concentration, a lower carbon monoxide pressure causes the
tube growth initiation process to proceed more slowly, allowing the
catalyst particle to become larger before the tube growth is
initiated. These larger catalyst particles spawn larger tubes.
Similarly, an increase in the metal concentration will allow
cluster formation to be more rapid, also resulting in the
production of larger tubes. The minimum size (5,5) tubes are
preferably formed under conditions where the tube growth initiation
is rapid relative to the catalyst cluster growth. By use of these
control mechanisms, SWNT tube diameter from (5,5) to about (10,10)
can be produced.
The third control mechanism, which involves addition of a
nucleation agent, such as Ni(CO).sub.4, which accelerates the
aggregation rate of catalyst clusters, will also result in an
increase in the diameter of the tubes produced. In addition, the
tube diameter can be controlled by varying the temperature in the
mixing zone. In general, higher temperatures result in smaller
tubes.
The Chemical Process
The interaction of the catalyst precursor with the carbon monoxide
initiates the formation of metal clusters via gas-phase reactions
in the presence of carbon monoxide. These interactions may involve
thermal energy transfer that induces dissociative processes in a
molecular precursor, interaction of the carbon monoxide with
dissociation fragments of a precursor molecule, attachment of one
or more carbon monoxide molecules to a precursor molecule fragment
or to a metal atom that serves as a precursor, and/or participation
of the carbon monoxide in processes by which the metal catalyst
particle aggregates. In the process of the present invention, metal
catalyst particles grow by aggregation in the gas phase.
At relatively high carbon monoxide pressure and a suitable
temperature, tube growth begins on catalyst particles after they
reach the minimum size required to support tube formation. The tube
growth proceeds by the Boudouard reaction
(CO+CO.fwdarw.C(SWNT)+CO.sub.2) on these Fe.sub.x catalyst
particles, forming a single-wall carbon nanotube on each particle,
the single-wall carbon nanotube continuing to grow with the
particle at its "live end." The high pressure of CO and 800
1000.degree. C. temperature insure that this reaction is fast and
that defects in the single-wall carbon nanotube are annealed away
as it is formed. The high pressure of CO is necessary to (1) insure
that every Fe.sub.x starts a single-wall carbon nanotube before the
Fe.sub.x particle has grown too large by addition of Fe atoms or
larger Fe clusters, and (2) to drive the equilibrium toward the
single-wall carbon nanotube+CO.sub.2 products even in the presence
of substantial CO.sub.2 partial pressures that develop as the
reaction proceeds. In this regard the new method for single-wall
carbon nanotube production disclosed here resembles the Haber-Bosch
process for the syntheses of ammonia
(N.sub.2+3H.sub.2.fwdarw.NH.sub.3) over an activated iron
catalyst.
The formation of metal atom catalyst clusters must take place
rapidly and at the place and time at which conditions are optimum
for initiation of the Boudouard reaction. Cluster size when the
growth reaction begins dictates the diameter of the nanotube. In
the present invention, the smallest tubes produced have diameters
of about 0.6 nm. There are reaction conditions under which this
tube diameter is more likely to be produced than other tube
diameter. The 0.6 nm dimension is the diameter of the (5,5)
nanotube, which is the same as the diameter of the C.sub.60
molecule
To prevent cluster overgrowth and reaction termination; all the
precursor molecules should be dissociated and used to make clusters
nearly simultaneously (i.e., over very short periods of time). If
large amounts of catalyst precursor species remain in the
environment with active clusters supporting nanotube growth, these
precursor species will aggregate on the active clusters, enlarging
them. As the diameter of the active cluster increases, so does the
probability that it will overcoat with a carbon coating, rendering
it inactive as a catalyst. Product from the process described here
contains many 2 3 nm. diameter metal clusters that are overcoated
with carbon, suggesting that growth to this size and overcoating
are the fate of all active catalyst clusters. This catalyst
deactivation mechanism is slowed if most of the catalyst precursor
species rapidly dissociate and their dissociation products form
active catalyst clusters.
Pyrolytic formation of amorphous carbon deposits on the growing
tubes and the reaction vessels is a known problem with most methods
for growing single-wall carbon nanotubes. In the present invention,
the production of undesired carbon forms is minimized because the
formation of free carbon from carbon monoxide is inherently a
process that occurs efficiently only with the action of a catalyst.
In the present process, an active catalyst is present only in the
form of metal clusters on the growing ends of single-wall
nanotubes.
As the key to high single-wall carbon nanotube production is to
keep the Fe.sub.x particles from growing too large, one observes
that the present process has an additional advantage in that the
catalyst quickly grow a long single-wall carbon nanotube, and
collisions between these particles (which would otherwise result in
coalescence to produce a much larger particle) are eliminated
because all such collisions are dominated by tube-tube encounters.
These tube-tube encounters can result in the colliding, growing
tubes coming into alignment in van der Waals contact. Even if the
tubes aggregate with each other or with other small "ropes", the
Fe.sub.x clusters at the end of each tube are then prevented from
coming into frequent contact, while remaining as "live" ends of
their respective single-wall carbon nanotube.
Apparatus Description
The apparatus schematically shown in FIG. 1 will now be described
in more detail with reference to. FIGS. 2(a) and 2(b) where like
numerals refer to the previously described elements. FIG. 2(a)
shows the oven 10 and the portion of the system of FIG. 1 that is
associated with it. Oven 10 is a cylindrical aluminum pressure
vessel containing electrical resistance heating element 11
surrounded by insulating material (not shown) in the central
portion. Other materials and heating methods can, of course, be
employed as is known in the art. Suspended in axial orientation in
oven 10 is reactor tube 20. This reactor tube 20 includes both the
mixing zone 8 and growth and annealing zone 9. In the illustrated
embodiment, tube 20 is quartz and has a diameter of 7.5 cm and a
length of 120 cm. Undiluted CO feed stream 3 in this embodiment
enters the oven 10 near the exit and is passed countercurrently
through conduit 21 at the periphery of the growth and annealing
zone 9 to supply CO to the mixing zone 8, shown in more detail in
FIG. 2(b). This conduit 21 is, in the illustrated embodiment, a
copper coil 21 of 0.250'' in O.D. spirally wound tubing. This
configuration employs the heat in the quartz tube (from the oven
and growth annealing zone) to preheat the CO gas stream fed to
mixing zone 8. This embodiment therefore is highly thermally
efficient.
Referring now to FIG. 2(b), a portion of the reactor tube 20 is
shown in the vicinity of mixing zone 8. The catalyst precursor/CO
stream 3 enters via stainless steel tube 22, which is water-cooled
by jacket 23. Tube 22, which in the illustrated embodiment is
0.260'' I.D., leads directly into axial flow nozzle 24 (also
quartz), which delivers the catalyst precursor/CO feed mixture
directly into mixing area 25. In the embodiment shown, nozzle 24
has a 0.260'' O.D. at its upstream end and 0.075'' O.D. at its
downstream tip. Its orifice I.D. at the downstream tip is
approximately 0.040''. The countercurrent undiluted CO flow in tube
21 is connected to manifold 26, through which nozzle 24 also
protrudes. Manifold 26 preferably can be formed of stainless steel,
graphite or the like. In manifold 26 the CO stream is redirected
tangentially to the axial flow from nozzle 24 and supplied to
mixing area 25 through a plurality of radially disposed
tangentially directed injectors 27. In the illustrated embodiment
these injector tubes have an outlet I.D. of 1 mm, and are holes
bored through the body of manifold 26. Any number of such
tangentially directed injectors may be employed. At least three
such injectors 27 are preferred. Any configuration of mixing area
and injectors that achieve the rapid mixing described above can be
employed. In the embodiment shown, the tangentially directed
injectors intersect the axis of the axial injector nozzle at an
angle of about 30.degree.. Other angles may, of course, be
employed.
FIG. 3 shows an alternative embodiment of the apparatus of the
present invention in which the undiluted CO feed and preheating are
positioned upstream of mixing zone 8. In this embodiment, CO is fed
to mixing zone 8 through feed tube 30 which, as shown, is a 12 mm
O.D. quartz tube. Preheating is effectuated by a suitable resistive
heating element 31, which in the illustrated embodiment is a
graphite rod in electrical contact with graphite manifold 26 and a
copper electrical return rod 32. Other forms of energy input for
preheating the CO may be employed in known manner. This
configuration operates similarly in all material respects to the
embodiment of FIG. 2, except that waste heat in the growth and
annealing zone 9 is not directly recovered to preheat incoming CO.
Heat recovery means may be employed in the CO recycle loop if
desired.
FIG. 4 shows an alternative embodiment of the present invention. In
this embodiment a high repetition rate pulsed laser (repetition
rate>1 kHz) is employed to supply some or all of the energy
needed for photolysis cluster precursors, i.e., to dissociate the
catalyst precursor and form active catalyst metal atom clusters. As
such, the provision of this laser input may be termed a nucleation
agency. As illustrated, the output beam of a KrF laser 40 is passed
through a quartz window 41 in oven 10 and focused to impinge on the
gas mixture in mixing area 25. The laser operates at a repetition
rate of 1000 pulses per second at a power level of 50 millijoules
per pulse. As in the other embodiments, the CO feed gas is
preheated to approximately 1000.degree. C. It is also possible to
employ a CW laser as the nucleation agency.
In an alternate embodiment, shown in FIG. 5, the process of the
present invention can be carried out in a two-part reaction zone.
In reaction initiation (nucleation) zone 11, CO is contacted with a
catalyst precursor under conditions that favor formation of the
proper size metal cluster on the end of a growing single-wall
nanotube (e.g., P.sub.1>10 atmospheres, T.sub.1=850.degree. C.
to 1250.degree. C.). In reaction growth zone 12, the conditions and
reactants are changed to favor growth of carbon nanotubes (e.g.,
introduction of C.sub.2H.sub.2 as an alternate carbon source
through inlet 13, and P.sub.2=1 atmosphere, T.sub.2=850.degree. C.
to 1250.degree. C.).
Another embodiment of the present invention is shown in two
variations in FIGS. 6 and 7. In its preferred form, this
alternative embodiment begins with a KrF excimer laser dissociation
of ferrocene (FeCp.sub.2) that has been premixed in high pressure
CO (10 1000 atm) and heated to a temperature of 800 1000.degree. C.
While a laser is used to initiate the catalyst formation, this
laser is one that is available for routine operation on an
industrial scale, and only moderate laser intensities (.about.100
mJ/cm.sup.3 in 25 ns pulses, 10 250 pulses per second) are
necessary. Although catalyst formation is stimulated by a laser,
the single-wall carbon nanotubes are grown from a cheap industrial
gas, CO, at moderate temperature in a continuous, easily-scaled
process. The laser may be directed in the downstream direction, as
shown in FIG. 6, in the upstream direction, as shown in FIG. 7, or
in a crossing direction as shown in FIG. 4.
The high thermal stability of ferrocene insures that little
decomposition of this gas phase molecule occurs while it is mixed
with the CO and reaches the desired operating temperature. The KrF
excimer laser then efficiently dissociates the ferrocene as it
exits the catalyst addition tube. The 5.0 eV KrF laser photons are
absorbed by the ferrocene molecules with an effective cross-section
of 5.times.10.sup.-18 cm.sup.2, resulting (at the 800 1000.degree.
C. temperature of the reactor) in prompt dissociation to produce a
FeCp.cndot. radical plus a cyclopentadienyl radical Cp.cndot.. Some
of these FeCp.cndot. radicals absorb a second Kr photon and
fragment further to Fe+Cp.cndot.. These laser-produced free
radicals attack the remaining undissociated ferrocene in chain
reactions resulting in the nucleation of Fe.sub.x clusters, which
in turn, also promote dissociation of the ferrocene. The choice of
CO is particularly useful since not only does the high pressure
provide frequent collisions which are necessary to thermalize the
clustering Fe atoms and FeCp.cndot. radicals, but it also complexes
with a substantial fraction of Fe (.about.20% for P.sub.CO=100 atm
at T=1000.degree. C.) to produce FeCO (thereby carrying "its own
third body" to take away the excess energy of binding as the Fe
atoms begin to cluster). The Cp.cndot. radicals from the laser
dissociation of the ferrocene react with one another and pyrolyze
to produce small carbon clusters that further aid the nucleation of
the Fe.sub.x catalyst particles and act as feedstock in the early
stages of single-wall carbon nanotube growth. So, in spite of the
great thermal stability of ferrocene, the KrF laser triggers an
avalanche of dissociation and clustering events that, within a few
microseconds, produces a high number density of .about.1 nm
diameter catalyst particles.
The method described above is not restricted to ferrocene. Other
metallocenes, such as ruthenocene, cobaltocene, etc., may be used
as well as the carbonyls such as Fe(CO).sub.5, Mo(CO).sub.6, etc.,
as well as combinations of these with each other and with other
volatile organometallics. All these species have strong absorption
for KrF excimer wavelengths. Alternatively, other laser wavelengths
may be used to dissociate the organometallic catalyst precursor
module. For example, the ArF excimer laser wavelength is strongly
absorbed by ferrocene (cross-section for absorption
.about.10.sup.-16 cm.sup.1) and results in prompt dissociation, but
the "Cameron Bands" absorption of CO at this wavelength will
attenuate the ArF excimer laser beam as it propagates through the
reaction oven.
Catalyst promoters, such as thiophene, H.sub.2S, or volatile lead
or bismuth compounds may be added to the CO as well to fine tune
the activity of the catalyst and/or the diameter distribution of
the single-wall carbon nanotube product.
Product Description
The product of the present invention is a composition that
comprises single-wall carbon nanotubes and/or ropes of these
materials (i.e., up to 10.sup.3 tubes generally aligned and held
together by van der Walls forces). The compositions, as produced,
are extremely clean and can be used directly without expensive and
time-consuming purification steps. In the preferred product of this
invention these compositions are substantially free of amorphous
carbon and contain only minor amounts of catalyst atoms as
impurities. The compositions of the present invention can contain
greater than 75% of SWNTs. The preferred products according to this
invention may comprise greater than 99% SWNTs. These percentages
are on an atom basis.
Another important aspect of the products of the present invention
is the unique tube diameter properties of these compositions. The
SWNT compositions of this invention provide tube diameters that are
smaller than products produced by prior art processes. In general,
the tube diameters of the products of the present invention are in
the range of frOm about 0.6 nm to about 2 nm. The preferred
products of this invention have tube diameters in the range of from
about 0.6 nm to about 0.8 nm. Compositions according to this
invention will have greater than 50%, preferably greater than 75%,
and most preferably, greater than 95%, of all SWNTs in this 0.6 nm
to 0.8 nm diameter range. Moreover, by the control mechanisms that
form a part of this invention, it is possible for the first time to
produce products with substantial quantities of (5,5) tubes.
The 5,5 tube is one of the smallest, if not the smallest, diameter
stable single-wall nanotube that can be formed, and of all (n,n)
tubes, its sidewalls should be the most chemically active because
they are the most strained. In general, products that comprise at
least 25% (5,5) tubes and preferablY those that comprise at least
50% (5,5) tubes are provided by the present invention.
The products of the present invention can be seen in FIG. 8. FIG.
8(a) is a TEM that shows the individual tubes in the product. FIG.
8(b) is a SEM that shows a mass of ropes of tubes in the 0.6 nm to
0.8 nm diameter range.
Carbon nanotubes, and in particular the single-wall carbon
nanotubes of this invention, are useful for making electrical
connectors in micro devices such as integrated circuits or in
semiconductor chips used in computers because of the electrical
conductivity and small size of the carbon nanotube. The carbon
nanotubes are useful as antennas at optical frequencies, and as
probes for scanning probe microscopy such as are used in scanning
tunneling microscopes (STM) and atomic force microscopes (AFM). The
carbon nanotubes may be used in place of or in conjunction with
carbon black in tires for motor vehicles. The carbon nanotubes are
also useful as supports for catalysts used in industrial and
chemical processes such as hydrogenation, reforming and cracking
catalysts. The nanotubes may be used, singularly or in multiples,
in power transmission cables, in solar cells, in batteries, as
antennas, as molecular electronics, as probes and manipulators, and
in composites.
EXAMPLES
In order to facilitate a more complete understanding of the
invention, Examples are provided below. However, the scope of the
invention is not limited to specific embodiments disclosed in the
Examples, which is for purposes of illustration only.
Example 1
This Example employed the apparatus shown in FIGS. 1 and 2 and
demonstrates the process of the present invention is useful to
produce clean SWNTs of small diameter.
Summary of Conditions:
TABLE-US-00001 Operating Pressure: 600 psi (40 atmospheres) of CO
Operating Temp. 900.degree. C.
Flow Conditions:
Two standard liters per minute (slm) of CO containing 0.5 Torr of
Fe(CO).sub.5 were passed through the air-cooled injector. 8 slm of
pure CO were preheated in the copper heating coil, and passed
through the stainless steel injector manifold. The two flows were
mixed in the mixing zone and the combined gases passed through the
growth and annealing zone and into the product recovery zone.
Run time: 2 hours
Results:
17.5 mg of material was collected from the product recovery zone at
the exit of the high pressure reactor. SEM measurements showed that
this material was primarily SWNT. EDX and TGA measurements showed
that this material contained 3 5 atom % of iron. TEM measurements
showed that the narrowest single-walled nanotubes in this product
were 0.7 nm in diameter, corresponding to the expected size of a
(5,5) carbon nanotube.
Example 2
Using the same apparatus as in Example 1, this Example demonstrates
that Ni(CO).sub.4 does not exhibit appreciable catalytic effects
under the preferred higher pressure CO process conditions.
Summary of Conditions:
TABLE-US-00002 Operating Pressure: 450 psi (30 atmospheres) of CO
Operating Temp. 1000.degree. C.
Flow Conditions:
2.5 standard liters per minute (slm) of CO containing 0.4 Torr of
Ni(CO).sub.4 were passed through the air-cooled injector. 7.5 slm
of pure CO were preheated in the copper heating coil, and passed
through the stainless steel injector manifold. The two flows were
mixed in the mixing zone and the combined gases passed through the
growth and annealing zone and into the product recovery zone.
Run time: 2 hours
Results:
Powdery material was collected from the product recovery zone at
the exit of the high pressure reactor. This material was not
weighed. SEM measurements showed that this material contained no
SWNT; it was composed of metal particles overcoated with
carbon.
Example 3
Again using the apparatus of Example 1, this Example shows that
employing Ni(CO).sub.4 as a nucleating agent substantially improves
the productivity of the high pressure CO process.
Summary of Conditions:
TABLE-US-00003 Operating Pressure: 450 psi (30 atmospheres) of CO
Operating Temp. 1000.degree. C.
Flow Conditions:
2.5 standard liters per minute (slm) of CO containing 0.2 Torr of
Fe(CO).sub.5 and 0.2 Torr of Ni(CO).sub.4 were passed through the
air-cooled injector. 7.5 slm of pure CO were preheated in the
copper heating coil, and passed through the stainless steel
injector manifold to the mixing zone where it was combined with the
injector flow. The combined gases passed through the growth and
annealing zone into the product recovery zone.
Run time: 2 hours
Results:
20.1 mg of material was collected from the product recovery zone at
the exit of the high pressure reactor. SEM measurements showed that
this material was primarily SWNT. EDX measurements showed that this
material contained 1.2 atom % of iron and 0.6 atom % of nickel. TEM
measurements showed that the single-walled nanotubes in this
product were 0.8 nm in diameter. Under similar conditions employing
only Fe(CO).sub.5, the yield was 3 4 times lower than in this
Example.
Example 4
Referring to FIG. 6, the high pressure CO reaction chamber is made
of a 2'' diameter, 42'' long quartz tube inserted through a 3-zone
furnace mounted within a 16'' O.D., 11'' I.D. aluminum cylinder
with 3'' thick aluminum end flanges. The inside of the quartz tube
is maintained at the 10 100 atm operating pressure of CO by control
of the mass flow controllers for the main gas flow, and the
catalysts addition stream, and by adjusting a throttle valve at the
exit. The inside of the aluminum pressure tank is pressurized with
inert gas (preferably Ar) so that the external pressure around the
quartz reactor tube is never greater than the inside CO pressure by
more than 10 psi, nor less than 5 psi. This is accomplished with a
differential pressure regulator.
Ferrocene is added through the catalyst addition tube. This is a
0.5'' diameter quartz tube with a 5 mm wide exit hole at the end,
arranged so as to direct the ferrocene containing CO flow (.about.1
liter/min) upwards into the oncoming (.about.10 liter/min) flow of
CO in the main portion of the 2'' reactor tube. Ferrocene is
sublimed from a separately heated section of this addition tube
just before it enters the main oven of the high pressure reactor.
The partial pressure of ferrocene (0.01 to 0.1 Torr) is controlled
by the temperature of this sublimation zone (100 200.degree. C.).
As shown, the unfocused beam of a KrF excimer laser (300 mJ/pulse
in a 1.5 cm.times.3 cm rectangular beam profile, 30 pulses per
second) is directed down the axis of the quartz tube reactor,
passing just above the exit of the catalyst addition tube. The
product single-wall carbon nanotube is collected on the cool walls
of the quartz reactor tube and on in-line filters as the CO gas
exits the oven. The CO.sub.2 produced 15 as a result of the
Boudouard reaction is removed from the exiting CO gas by a reactive
filter. The purified CO gas is then recompressed, purified a final
time to remove H.sub.2, hydrocarbons, transition metal carbonyls,
etc., and recirculated to the quartz reactor tube.
Example 5
An alternative design for large throughput operation may be
achieved by having the reactant CO+ferrocene gas flow at high
velocity perpendicular to the KrF excimer laser, thereby allowing a
large volume to be excited in a single laser pulse. This utilizes
the ability of modern KrF lasers (e.g., Lambda Physik model LPX
325i) to operate at 250 pulses per second, each pulse interacting
with yet a new volume of gas. At an initial ferrocene partial
pressure of 0.1 Torr, a single laser pulse propagates usefully
through a meter of the CO reactant gas, nucleating Fe.sub.x
catalyst particles uniformly in a 1 liter volume (assuming a 10
cm.sup.2 beam profile). At a large flow velocity of 750 cm per
second in the irradiation zone, this 250 Hz laser activates 250
liters per second for efficient single-wall carbon nanotube growth.
Assuming every Fe.sub.x catalyst nucleates a single-wall carbon
nanotube of average length of 10 microns, and assuming that most of
the Fe in the initial ferrocene becomes involved in such a catalyst
particle, Fe.sub.x, with x.about.100, this means that roughly 0.1
kg of single-wall carbon nanotube is produced every second in the
final collected product downstream. A single-wall carbon nanotube
production unit operating just this one KrF laser can be able to
deliver several tons of single-wall carbon nanotube per day in
continuous operation.
In order to keep the reactant gas temperature under control as the
single-wall carbon nanotube are formed in the reactant gas (for
Boudouard reaction .DELTA.H=-170 kJ per mole of carbon) it is
useful to expand the reacting gas by a factor of .about.10 after
the initial laser nucleation zone is passed. As the gas accelerates
toward this expansion point (effectively a long slit nozzle), the
desired flow velocity of 750 10 cm/sec will be easily achieved.
After the single-wall carbon nanotube are nucleated and
well-established in growing ropes, the need for high pressure CO is
largely over. Subsequent growth can then proceed at lower rates in
the lower pressure CO, giving enough time for the gas to cool by
radiation (the single-wall carbon nanotubes are excellent black
body emitters) and by heat exchange with the walls.
The presence of CO.sub.2 at a pressure near the thermodynamic
equilibrium point will help to eliminate defects from the growing
single-wall carbon nanotube. For this reason it may be useful to
seed the initial CO gas with a small amount of CO.sub.2, and/or
inject it downstream of the KrF laser irradiation/nucleation
zone.
Example 6
Referring to FIG. 7, by changing the laser beam from the downstream
direction to the upstream direction, a "cold injector" for the
incoming ferrocene coming in from the upstream direction mounted on
the central axis of the quartz tube may be used. The hot CO shower
head may be used, but it may be desirable to use a preheater coil
for this shower head so that the downstream walls remain clean.
With a larger quartz tube, an all-metal "cold injector" may be used
with air cooling that achieves ferrocene sublimation at
temperatures in the range of about 90 150.degree. C., and an exit
temperature of about 500.degree. C. with about 2 3
thermocouples.
On the downstream end of the apparatus, a quartz window is provided
for the laser input. A CO purge flow may be necessary to keep this
window clean. There is also a need for a collector for the SWNT
deposits. This may be achieved with a water-cooled copper cylinder
mounted in the quartz tube as the flow exits the oven that also
serves to cool the 1000.degree. C. CO. In one embodiment, a 1.5''
O.D. copper pipe with about 5 to about 10 turns of 1/8'' copper
tubing brazed to the outside, with cold water circulating inside
the copper tubing, is used.
The production resulting from use of the upstream laser may be
limited by the creation of single-wall carbon nanotube "fuzz balls"
that flow into the laser beam, slightly attenuating the laser beam.
At high yield, and at high production rate, this shadowing will be
self-limiting.
The upstream laser, however, will interact with any ferrocene
molecules that would otherwise had the chance to fatten the
catalyst particles on the still-growing single-wall nanotube
product.
While the invention has been described in connection with preferred
embodiments, it will be understood by those skilled in the art that
other variations and modifications of the preferred embodiments
described above may be made without departing from the scope of the
invention. Other embodiments will be apparent to those skilled in
the art from a consideration of the specification or practice of
the invention disclosed herein. It is intended that the
specification is considered as exemplary only, with the true scope
and spirit of the invention being indicated by the following
claims.
* * * * *